Scanning Coils

Abstract

A circuit element, which is intended to be rolled into a cylindrical or like form to constitute a scanning coil assembly for a cathode-ray tube, is formed of a tapelike, flexible substrate with conductive patterns on either side thereof. Each pattern comprises a plurality of side by side conductors conforming to a longitudinally extending, substantially rectangular waveform, formed of alternate longitudinal and transverse portions. The longitudinal portions of the two waveforms are staggered with respect to each other, and respective conductors in the transverse portions are offset with respect to one another in the longitudinal direction so as to tend to reduce the self-capacitance of the element when wound to form a scanning coil assembly.

Description

This invention relates to scanning coils for cathode-ray tubes and in particular to printed circuit scanning coils.

In the specification of U.S. Pat. No. 3,466,580 there is described and claimed a printed circuit scanning coil and a circuit element used in producing such coils. According to said specification there is claimed a circuit element comprising a flexible base which supports two overlapping and mutually insulated electrically conductive patterns, each pattern comprising a repetitive longitudinally continuous substantially rectangular waveform, consisting of alternate longitudinal and transverse parts at right angles to each other, one pattern being longitudinally displaced relative to the other so that the longitudinal parts of the respective waveforms are staggered so that the base can be rolled in such a way as to cause the patterns to form the equivalent of two diametrically opposed scanning coils, wherein the transverse parts become parallel to the coil axis and the longitudinal parts extend circumferentially of the coils.

In one practical form of the invention, the two conductive patterns are provided initially on the same side of a thin insulating flexible base. The patterns, formed say by copper adhering to the base, can be produced by a process such as may be used for forming printed circuits. The thin flexible base is then cut along its longitudinal centerline, except when there are conductors joining the two conductive patterns, the two halves are folded so that the one pattern lies on top of the other, and the base is then rolled. A thin sheet of flexible insulating material is inserted to ensure that the two patterns do not contact in the finished coil. In the finished coil the conductors in adjacent layers of the roll lie very close together and it is found that this may produce appreciable capacitance.

The object of the present invention is to reduce this effect and in accordance with the invention a printed circuit scanning coil arrangement comprises a flexible base which supports two overlapping and mutually insulated electrically conductive patterns, interconnected to form a series circuit, each pattern comprising a plurality of side by side conductors conforming to a repetitive, longitudinally extending rectangular waveform consisting of alternate longitudinal and transverse parts at right angles to one another, one pattern being displaced relative to the other so that the longitudinal parts of the respective waveforms are staggered, the base being rolled in a plurality of turns about an axis parallel to said transverse conductor parts, to form scanning coils in which the transverse parts of the conductors are disposed in two diametrically opposite groups, the improvement lying in that the transverse parts of the conductors of the two patterns are relatively displaced in the longitudinal direction to offset the respective parts of the conductors in adjacent turns, thereby to reduce the capacitance of the coils.

Preferably, the spaces between the conductors are substantially equal to the widths of the respective conductors and the conductors are arranged so that when the base is rolled the conductors in one layer are superimposed on the spaces between the conductors on the next adjacent layer.

In order that the present invention may be clearly understood and readily carried into effect it will now be described with reference to the accompanying drawings in which:

FIG. 1 illustrates diagrammatically a simplified form of a circuit element used in forming a scanning coil according to one example of the present invention,

FIG. 2 is a perspective view of the finished coil mounted on a cathode-ray tube,

FIG. 3 illustrates a fragment of a circuit element used in one practical form of the invention,

FIG. 4 is a diagrammatic representation of the scanning coil utilizing the circuit element of which a fragment is illustrated in FIG. 3, showing the relative position of the conductors, and FIG. 5 shows, on an enlarged scale, a portion of the right-hand end of FIG. 1, and an alternative configuration for the conductors.

Referring to the drawings, the circuit element shown in FIG. 1 comprises a thin flexible base 1 formed of insulating material such as Mylar. Two conductive patterns 2 and 3 are formed on the same surface of the base 1, each pattern comprising a repetitive longitudinally extending rectangular waveform. For the sake of simplicity only four conductors are shown in each pattern and each pattern includes only four cycles of the rectangular waveform, although in practice, a larger number of conductors and cycles may be required. Two terminals 4 and 5 are formed at one end of the base 1 and the conductors are connected successively one to the other by connecting conductors 6 and 7 to form a single series circuit from the terminal 4 to the terminal 5. The material of the conductors forming the patterns 2 and 3 is copper adhering to the base 1. The patterns are formed by starting with a base 1 which is uniformly coated with a thin copper layer and by etching away unwanted copper as is done in the preparation of printed circuit boards.

To complete the formation of a scanning coil from the circuit element shown in FIG. 1, the base is cut along the centerline 8, except where the connecting conductors 6 and 7 are provided, and folded so that the pattern 3 is superimposed on the pattern 2, the patterns being on what are now the two outer surfaces of the base. The longitudinal parts such as 9 and 10 of the patterns are now staggered but the transverse parts are superimposed. The base is rolled to form a cylinder, the diameter of which is such that the transverse parts of the patterns form two diametrically opposite groups. To preserve the correct alignment of the conductors in the successive layer of the roll, the pitch of the patterns 2 and 3 must increase progressively in proportion to the winding radius. To assist in achieving accurate registration, registration strips of copper 11, 12, 13 and 14 are printed on the base alongside the active conductors of the patterns 2 and 3, the strips containing small holes spaced at what will be equiangular intervals when the base is rolled. When winding occurs a jig with radial pins engages the holes in the copper strips and ensures that there is no cumulative error in position. The two patterns 2 and 3 are also formed to slightly different scales to allow for the fact that they will lie at different radii of the roll. During rolling, a thin sheet of insulating material is inserted to prevent contact between the copper conductors. The roll is impregnated with adhesive to form a self-supporting unitary coil structure, the registration strips 11 to 14 being eventually cut away leaving a finished coil such as represented in FIG. 2.

The coil is dimensioned to suit a particular cathode ray tube and FIG. 2 shows the coil fitted to a cathode-ray tube in the form of a television pickup tube 15 having a photoconductive target. In practice, line and frame coils may be printed on a single base, together with electrostatic screens which separate and enclose the coils. As shown in FIG. 1, the connecting conductors 6 and 7 are inclined to allow for the different scales of the two patterns 2 and 3. When the base is folded after cutting along the centerline 8 but before rolling, the uncut connecting pieces of insulating material, to which adhere the connecting conductors 6 and 7, may be corrugated to prevent wrinkling as described in the specification of U.S. Pat. No. 3,466,586. The connecting pieces may also be strengthened with adhesive tape.

As so far described, the coil illustrated in FIGS. 1 and 2, though differing in practical detail, is generally similar to that described and claimed in the specification of U.S. Pat. No. 3,466,580. The illustrated coil differs however in that the conductors forming the patterns 2 and 3, though shown as relatively thin in FIG. 1 are of substantially the same widths as the spaces between them. This can be seen in FIG. 3, which shows a fragment of the two patterns each consisting of nine conductors 2.sub.1 to 2.sub.9 and 3.sub.1 to 3.sub.9. In the FIG. 3 the two patterns correspond respectively to the patterns 2 and 3 of FIG. 1 but the pattern 3, comprising the conductors 3.sub.1 to 3.sub.9 is seen looking through the base 1, after the base has been folded. However, it is also shown, for illustration purposes, displaced from the correct position under the half of the base carrying the pattern 2. In practice after folding edge 2a is adjacent the edge 3a. In FIG. 3 the registration strips 11 to 14 are omitted. FIG. 3 shows that the transverse parts of the conductors of the pattern 2 are displaced from those of the pattern 3 in the longitudinal direction in such a way that the conductors of different layers are offset. The displacement is such that when the base has been rolled, the conductors 3.sub.1 to 3.sub.9 are superimposed on spaces between the conductors 2.sub.1 to 2.sub.9 on the next adjacent layer. This is represented diagrammatically in FIG. 4. It will be appreciated that to achieve this superimposition the relative displacement of the conductors 2 and 3 on the flat circuit element must differ slightly from half the distance between two adjacent conductors 2 (or two adjacent conductors 3) to allow for increasing radius. No relative displacement of the longitudinal parts of the conductors is required, since the longitudinal parts of the different patterns are not superimposed when the base 1 is rolled. In one practical coil, the use of the invention achieved a reduction of the self-capacity of the coil from around 1200 pf. to around 425 pf.

It is also desirable to form the conductors which make up the patterns 2 and 3 in such a way as to reduce eddy currents in the conductors. Therefore the conductors may be interrupted by one or more gaps. Alternatively each conductor may be constituted by a number of thin parallel strands connected together at the terminals, as shown in FIG. 5, only four sets of conductors being shown in each of patterns 2 and 3 for simplicity. Each set of conductors is formed of three parallel and closely spaced conductor portions, these portions being separate apart from being interconnected at the terminals 4, 5. In FIG. 5, all features which are common to FIG. 1 are given the same reference numerals, and FIG. 5 represents, on an enlarged scale, the right-hand end portion of FIG. 1, with the difference that sets of parallel conductor portions are employed instead of unitary conductor portions.

Claims

What we claim is:

1. A printed circuit scanning coil arrangement comprising a flexible base which supports two overlapping and mutually insulated electrically conductive patterns, interconnected to form a series circuit, each pattern comprising a plurality of side-by-side conductors conforming to a repetitive, longitudinally extending rectangular waveform consisting of alternate longitudinal and transverse parts at right angles to one another, one pattern being displaced relative to the other so that the longitudinal parts of the respective waveforms are staggered, the base being rolled in a plurality of turns about an axis parallel to said transverse conductor parts, to form scanning coils in which the transverse parts of the conductors are disposed in two diametrically opposite groups, the improvement lying in that the transverse parts of the conductors of the two patterns are relatively displaced in the longitudinal direction to offset the respective parts of the conductors in adjacent turns, thereby to reduce the capacitance of the coils.

2. A printed circuit scanning coil arrangement according to claim 1 in which said relative displacement of the transverse parts of the conductors is such that the transverse conductor parts in one turn of the coils are superimposed on the spaces between the conductors on the next turn of the coils.

3. A printed circuit scanning coil arrangement according to claim 1 in which the width of the transverse parts of said conductors is substantially the same as the width of the spaces between the transverse parts of said conductors.